Emissions Savings' Potential of Wind and Solar Power

Tuesday, 10 December 2013

REF is often asked about the lifetime emissions saving potential of uncontrollable renewables such as wind and solar, the output of which is difficult to predict with great accuracy even a few hours ahead.

Given uncertainties about the embedded emissions in site specific applications of these technologies, which may vary considerably (due to difficult access or disturbance of peat, for example), it is inherently very difficult to give a generally adequate answer.

Moreover, it is not clear how solar and wind generators interact with the conventional plant in the rest of an electricity system. To be specific, there are uncertainties a) as to which conventional plant is likely to be displaced by wind and solar, and, b) if it is fossil-fuelled plant, whether the thermal efficiency of plant is significantly degraded by the ramping of output required when operating in the support role.

In relation to the first of these points, the plant likely to be displaced, REF has consistently suggested that the uncertainties are such that analysis should presume only a range of values for emissions saved, from the lowest, associated with gas, to the highest associated with coal, with a grid average emissions factor being used for approximate quantitative calculations. However, it should be recognised that the grid average emissions may not reflect the most probable displacement scenario at any one time.

Careful examination of the GB fuel mix data (published by REF) in recent months confirms this approach, and indicates that gas, which is currently the plant that ramps up and down most to meet consumer demand, is also the plant that is most likely to be displaced by wind. Only a statistical analysis could confirm this claim, but the indicative evidence is highly suggestive.

Note the fact that from period 19 to period 31 load on the system was steady at approximately 43 GW. Output from coal and nuclear is steady over the period, while gas declines significantly between periods 27 and 31, a decline that corresponds with a significant increase in output from wind.Since load and coal and nuclear output are all stable over this period, it is reasonable to infer that the decline in gas generation is a response to the increase in wind output. In other words that wind power was displacing gas at this time.

This effect can be more clearly seen in the following plot which shows gas and wind contributions alone.

Such data confirms our view that when assessing the likely emissions savings from a wind project, particularly in the planning system where benefits must be weighed against disbenefits, coal displacement should be regarded as an unlikely outcome, with gas being the likeliest displaced fuel, with grid average emissions savings being used only as a very rough rule of thumb.

Here is a worked example for a 2.3 MW wind turbine working at 27% load factor.

2.3 MW x 8760 (hours in a year) x 0.27 = 5,440 MWh

Assuming emissions displacement at the grid average in 2011, which was 0.45 tCO2/MWh, this would save about 2,500 tonnes of CO2 per year.

The emissions per MWh of coal and gas vary from plant to plant, and according to character of operation, but figures of 0.95 tonnes CO2 per MWh for coal (though supercritical coal will perform better), and 0.4 tonnes CO2 per MWh for the UK's current combined cycle gas turbines (though the more modern Combined Cycle Gas Turbines currently now entering the market will perform better), give an approximate sense of the relative emissions (this is discussed at some length on page 15ff of David White's 2004 study for REF.

Thus if our sample wind turbine were to displace coal it would save about 5200 tonnes of CO2 per year; whereas it were to displace gas generation it would save about 2,200 tonnes per year.

These are obviously significant differences in magnitude, and have very significant effects on the wind power subsidy cost per tonne saved.